Does light have an influence on fish growth?

Light compares a complex of external and ecological factors, including colo ur spectrum, intensity and photoperiod. Light characteristics are very spec ific in an aquatic environment and light is extremely variable in nature. ' Receptivity' of fish to light profoundly changes according to the species a nd the developmental status. Specific photoreceptor cells are present in bo th eye and pineal. If it is easy to change the light in experimentation and to observe the effects on fish growth, it is much more difficult in nature to make such determinations. In larvae, many studies have been dedicated t o the influence of intensity and photoperiod on growth: generally, species need a minimal threshold intensity to be able to develop normally and grow. This is probably related to the aptitude to localize, catch and ingest pre y. Light is also indispensable for body pigmentation, an important phenomen on involved in early development and growth. Too intense light can be stres sful or even lethal. A few species are able to develop and grow at very low intensities or, sometimes, in the absence of light. Generally, long daylen gth improves larval rearing quality. The synergistic effect of 'food availa bility-daylength' appears to be determining at this stage. In older fish, t here is very little information about the influence of light 'quality' but more about intensity and much more about photoperiod. Light intensity effec ts are not so clear and depend on the species and the experimental procedur es: it is probably not an important factor for growth stimulation. Daylengt h appears much more important. Many species, including both marine species and salmonids, react to photoperiod treatments and long daylength stimulate s growth. The most studied species is the Atlantic salmon, which is very se nsitive, both during the freshwater stage, with the parr-smelt transformati on very dependent on the photoperiod, and also in sea water. In this last c ondition, lighting also influences early maturation. An important point is to be certain that light affects fish growth through a better food conversi on efficiency and not just through stimulated food intake. Also included in this review is a discussion about the endolymph-otolith system, which is v ery sensitive to daylight and seasonal cycles and a review of the present k nowledge on the involvement of light influence on hormone levels (melatonin , somatotropin, thyroid hormones and other hormones). (C) 1999 Elsevier Sci ence B.V. All rights reserved.